A prior art direct detector, is shown by way of example in FIG. 1. A direct detector detects the signal from variations in the signal amplitude, as opposed to variations in frequency. As shown, a light wave is incident at a photodetector 10, shown for purposes of illustration as a photodiode. The resulting electrical current output from detector 10 may pass through a pre-amplifier 20 such as shown, typically used to amplify the electrical signal to an intermediate level without degrading the signal to noise ratio of the signal. Thereafter, the signal is further amplified as it passes through post amplifier 30 for large amplification of the voltage for input to decision circuit 40. At this point the voltage is split along two paths. One path leads to a decision circuit 40 for data recovery, while the other path leads to a clock recovery device 50 for correcting the timing of the data signal which may have become distorted during transmission. The clock is split at power splitter 60 in order to provide timing information to decision circuit 40 and for external use.
The amplification stage after detection is necessary because the incident signal may be at a low power level on the order of 50 microwatts or lower, which will generate a low voltage on the order of 15 millivolts. The logic circuit, however, typically requires 1 volt peak to peak. The drawback of the device of FIG. 1 is that it includes electrical connections and devices. The electrical components are sensitive to data transmission rates and should have sufficient electrical bandwidth to operate at the data transmission rate. As bit rates increase, obtaining amplifiers with the proper bandwidth becomes a challenge. While connections can be fabricated to accept the higher bit rates, such as in the range of gigabits, they must also be compatible with the amplifiers they inter-connect which traditionally have impedance ratings of 50 ohms. Accordingly, in light of ever increasing demand for higher data transfer rates, it is desirable to reduce the constraints created by the electrical elements in the circuit, e.g. transistors, capacitors and wiring.